2008 S.C. Water Resources Conference - Addressing Water Challenges Facing the State and Regio

Hougham, Andrea L

Wilson, Alicia M

English

Clemson University: TigerPrints

6. Mean Sea LevelNOAA Tide Data, Charleston, SC (8665530)4. Stratigraphy3.  Study SiteNorth Inlet-Winyah Bay NERRSemi-diurnal tide, average amplitude 1.4 mLow marsh populated primarily by medium and tall form SpartinaalternifloraGroundwater Dynamics in a Tidal Salt Marsh: A Combined Field and Modeling StudyNorth Inlet, Georgetown, South CarolinaAndrea L. Hougham and Alicia M. Wilson University of South Carolina, Department of Geological SciencesAcknowledgements:We would like to thank Dr. L. Robert Gardner, Dr. James T. Morris, Dr. Willard S. Moore, Dr. Scott White, Karen Sundberg, Ipsita Gupta, Nickles Badger, Curtis Gebhard, Katrina Byerly, Weihong Wang, Whitney Pate, and Joseph Bell.Funding was provided by: -1-0.500.511.522.533.510/2/2007 10/5/2007 10/8/2007Feet above Marsh Surface4m2m1mTideNS2-1-0.500.511.522.533.510/2/2007 10/5/2007 10/8/20074m2m1mTideEW41. IntroductionSalt marsh groundwater dynamics provide important controls for: submarine groundwater discharge and associated solute fluxes the subsurface distribution of solutesporewater redox states.In the southeastern United States, areas of acute marsh dieback developed coincident with a period of severe drought in 2001 –2002, suggesting a link to hydrology. Acute marsh dieback is defined by its rapid development and sharp boundaries between dead plants and healthy marsh, further suggesting geologic and structural differences between dieback and unaffected marsh sites.2. HypothesesThis experiment was designed to test the following hypotheses: Stratigraphy below the dieback site is variable and affects groundwater flow dynamics and discharge locations. The dieback site elevation is lower than the surrounding marsh. There were unusual mean sea level changes coincident with the drought years of 2001-2002.http://www.northinlet.sc.edu/map.htm diebackNS3EW1 NS2 EW3 EW4NS1NImage Courtesy of Google Earth30 mEW2Fig. 1a Fig. 1bFig. 1a: Map of the North Inlet-Winyah Bay National Estuarine Research Reserve with dieback sites as red dot and star.Fig. 1b: Satellite image of study site showing location of piezometer nests. The dieback area is within the circle.Fig. 2bFig. 2aFigs. 2a and 2b: Stratigraphic cross-sections of the north-south  and east-west transects. Red blocks indicate locations and lengths of piezometer screened intervals. Sand layer is highlighted in tan.-1.5-1-0.500.511.522.58/6/2007 8/10/2007 8/14/2007 8/18/2007 8/22/2007Feet above Marsh Surface4m2m1mTideEW1-5-4-3-2-101238/6/2007 8/10/2007 8/14/2007 8/18/2007 8/22/2007Fig. 3a: Hydraulic head record from Nest EW1. showing vertical gradient indicating downward flow. The red circle highlights one of the  24-hour periods during which the marsh was not inundated. Fig. 3b: Tidal record corresponding to Fig. 3a. NOTE:the vertical hydraulic gradient increases  during extreme low tides.Figs. 4a and b: Hydraulic head record at Nests NS2 and EW4. The lack of vertical gradient at NS2 indicates stagnant flow conditions, and the vertical gradient at EW4 is greater than EW1 as a result of the increased thickness of sand.Fig. 3aFig. 3bFig. 4a Fig. 4bVibracores collected at each nest sitePressure and Temperature dataloggers deployed to calculate hydraulic head5. Elevation DifferencesLIDAR data collected 2003 (J. T. Morris)Fig. 5: The lowest elevation is indicated by the dark blue color. Colors change with increasing elevation from dark blue, light blue, green, yellow, orange, to red. The study site is circled and shows no difference in elevation from the surrounding marsh. 7. Exploratory ModelUSGS SUTRA Model (Voss, 1984)Fig. 7: Snapshot at low tide from exploratory model results. Note that the vertical gradient in hydraulic head is greater on the east side of the marsh—corresponding well to the field data.EW-1.00.01.02.03.04.05.06.07.0Feet above MLLWMean Sea LevelMean High TideMean Low TideFig. 6: January 2002 monthly mean sea level was lowest since 1990 (NOAA), coincident with the drought. The tide record from the Charleston, SC gauge correlates to Oyster Landing  (North Inlet) with an average r2 = 0.8.8. ConclusionsVariable StratigraphyHydraulic gradient is greatest on the eastern marsh edge corresponding to thicker sands at that locationMarsh Surface ElevationSite elevation is not lower than the surrounding marshTidal InfluenceApproximately 5% of tidal record  no inundationDrainage is enhanced at low tide extremesMonthly mean sea level in January 2002 lowest since 1990, coincident with drought periodExploratory 2-D ModelPreliminary hydraulic gradient variability corresponds well to field data

Groundwater Dynamics in a Tidal Salt Marsh: A Combined Field and Modeling Study North Inlet, Georgetown, South Carolina

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Groundwater Dynamics in a Tidal Salt Marsh: A Combined Field and Modeling Study North Inlet, Georgetown, South Carolina

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